Public endpoint allows anyone to obtain a client certificate for authentication. Features: - Higher PoW difficulty than paste creation (24 vs 20 bits) - Auto-generates CA on first registration if not present - Returns PKCS#12 bundle with cert, key, and CA - Configurable via FLASKPASTE_REGISTER_POW Endpoints: - GET /register/challenge - Get registration PoW challenge - POST /register - Register and receive PKCS#12 bundle